The musical climate of the 1920s, when Copland came to maturity, was not favourable to the use of popular or traditional material or idioms in ‘serious’ composition. Nationalism was no longer a powerful force in the Western world. Increased respect for folk-musics as cultural relics inhibited composers from using them freely as composition sources. Popular music was associated with commercial exploitation and the wooing of a mass audience on the lowest common level, serious music with uncompromising devotion to Art for Art's sake. The leaders of revolutionary movements—Schoenberg, Stravinsky and Hindemith, each in different ways—upheld the negative principle that one must not walk where men had walked before. The melodic and harmonic idioms of serious and popular music were growing steadily apart, serious music concentrating its meanings, increasingly concerned with a microstructure in which every detail was to be personal and expressive, popular music freely accepting melodic and harmonic cliché as an integral part of the language. Serious music rejected exact repetition as a means of driving home the message, popular music was heavily repetitive. Serious music often depended on intellectual construction-methods not to be appreciated by the unprepared listener; popular music demanded that all meanings should be immediately apparent.